Heres some additional info you might want to know.

1. How big is your tank:  Gal / litres

-It is 50 Gal

2. For how long has the tank been set up:

-It has been set up for almost 2 months

3. How many fish:

-4 goldfish

4. What kinf of fish:

-3 fantails and 1 common

5. What are the water parameters:    Ammonia.........ppm
                                                            Nitrite..............ppm
                                                            Nitrate.............ppm
                                                            pH..........................
                                                            Temp...............degrees Celsius / Fahrenheit

6. How often do you perform water changes:

-every week

7. How big are your waterchanges:      ..........about 20.......%

8. What kind of waterconditioner are you using:

-Tetra

9. Do you add beneficial bacteria:

-no

10. Symptoms your fish is showing:

- red vein in the tail

11. Since when does your fish show these symptoms:   

-few days ago

12. How is the normal behaviour of your fish:

- normal

13. How do the other fish behave:

-normal

14. Post pics of the sick fish:

15. Do you have a filter on your tank, what kind, how big (gallons per hour)

- yes 60 gallons

16. Do you provide extra aeration with an air pump and air stone/diffuser of some kind.

-yes i have a pump with an air stone

Thank you very much. The vein doesn't look inflamed so i'm not going to worry about it to much but i'm going to keep an eye on it just in case.